For convenience, the invention will be described with reference to a sheet of photographic material, but it is to be understood that the method and apparatus is also applicable to the measurement of the width of other material as described above.
For the accurate control of a photographic processing machine, it is necessary to replenish the processing baths to compensate for consumption of the chemicals therein as the photographic material is processed, and thus to maintain the chemical activity of the processing solutions. Improvements in the formulations of the processing solutions, and a desire to reduce the volume of liquid effluent which is produced, have lead in recent years to a gradual reduction in the rate and amount of replenishment required. This, in turn, has lead to a requirement for increasingly accurate control of the replenishment process. For black and white materials, the volume of replenishment solution required, for the developer and the fixer stages of the processing is a function of the area of the material processed and of the amount of developed image on the material. In many graphic arts, that is to say high-contrast black-and-white, processing machines, where a variety of material widths may be used, the area is usually approximately determined by measuring the sheet width and the length, for example by means of microswitches extending across the width of the entrance to the processor that are activated by the passing material. The length is determined by the time for which the switches are activated multiplied by the transport speed of the processor. The width is determined by the number of microswitches that are activated. For example, if there are four microswitches equi-spaced across the entrance to the processor and three are activated, then the width of the material relative to the maximum possible width must be between 40% and 80%, and the width is stated as 60%.+-.20%.
Another method of obtaining the width and length, and thus the image area, is to have the image exposing apparatus, for example, an image-setter, transmit this information to the processor, as is the case, for example with the Linotype Hell Herkules PRO/Advantage.TM. imagesetting system. The processor is then able to use the information from the image-setter to enable accurate replenishment and thus to maintain good process control. In the case of the Advantage.TM. processor, errors in the delivery of replenishment per unit area of film processed were reduced to about 5% of the volume delivered in order to maintain stability at the very low replenishment rates used.
U.S. Pat. No. 4,506,969 (Pako Corporation) discloses a film width and transmittance scanner system of a graphic arts film processor in which the transmittance of light through the film is measured along a line or set of parallel lines positioned at a skew angle with respect to the direction of film travel. Switches are employed to note passage of the leading and trailing edge of the film through the scanner. The scanner may employ a single light source and a single detector, each of which extends across the width of the film, but there is also disclosed use of a larger number of detectors extending sequentially across the width of the film in order to reduce the path length which the scanner adds to the processor. The scanner produces signals that are used in the control of a replenishment system of the processor.
U.S. Pat. No. 3,554,109 (Logetronics Inc) discloses an image monitoring and control system for determining the optical densities developed in sheets of image-bearing photosensitive material, for controlling the feeding of replenishment chemicals to a film processor. The system includes a scanner having twelve light communicating stations for sequentially receiving light from a corresponding plurality of sources that has passed through successive portions across the width of a developed film.